The present invention relates to a process for recovering benzene from a highly aromatic reformate containing olefins.
Processes for the production of benzene from hydrocarbon feedstock are well-developed in the art. In particular, one recent advance in benzene-producing technology in a process comprises contacting a hydrocarbon feed of ultra-low sulfur level with a platinum containing zeolite catalysts resulting in a highly aromatic product stream. U.S. Pat. No. 4,456,527, issued Jun. 26, 1984 to Buss et al. assigned to the assignee of the present invention discloses such a process, and is incorporated by reference herein.
Extraction processes, such as the "sulfolane" and UDEX are frequently used to extract aromatics from conventional reformate. These processes are liquid/liquid extractions and are effective when feed aromatics concentrations are usually below 50%. However, at aromatics concentrations above about 50%, liquid/liquid extraction is no longer effective or economically viable. A highly aromatic reformate with high benzene concentration and about 0.05 wt % to about 5.0 wt. % olefins is not recoverable using liquid/liquid extraction.
The customary methods, today, of attaining higher purities, are liquid phase extraction and extractive distillation. The first method is preferred when more than one aromatic compound, such as benzene and/or toluene and/or xylenes, is to be commonly recovered from aromatic feedstocks which contain relatively large quantities (more than 30% wt.) of non-aromatics, as is the case with the gasoline fraction from reformers. It was generally thought that extractive distillation offers economical advantages when only one aromatic is to be removed from a corresponding cut, if its content of non-aromatics is relatively small (below 30% wt.). The latter is particularly the case with feedstocks like hydro-refined coke oven benzole and hydrogenated pyrolysis gasoline from steam-crackers. These feedstocks contain toluene and xylenes typically only in small fractions, and often only benzene is recovered. The cuts containing toluene and xylenes are frequently admixed with motor fuels in order to increase the octane number.
Under some conditions, extractive distillation has been demonstrated to be an attractive benzene recovery process for streams having an aromatics concentration greater than about 35%. Extractive distillation processes have been used in the production of pyrolysis gasoline, or "pygas". However, pygas production processes include selective hydrogenation of olefins prior to the extractive distillation step, as the feedstreams typically contain high levels of olefinic compounds, which are undesirable.
In a hydrocarbon reforming and recovery process it is desirable to eliminate this hydrogenation step for economic reasons.
U.S. Pat. No. 3,434,936 issued Mar. 25, 1969 to Luther et al. describes a method of separating aromatic compounds from hydrocarbon streams by extractive distillation with an N-substituted morpholane. U.S. Pat. No. 3,434,936 also describes the problem of polymerization of compounds in the sump of an extractive distillation unit, and the need to hydrogenate the unsaturated compounds in the extract from the extractive distillation unit.
An improved process for the production of a highly pure benzene stream from a highly aromatic stream containing olefins is much desired.